The dramatic announcement effectively ends a campaign that took Cain, a former pizza executive who rose from humble origins, to the front ranks of a Republican field despite his lack of political experience.

Allegations

Cain's decision followed a drumbeat of accusations that erupted early last month when two women came forth to charge that he sexually harassed them while he was president of a trade lobbying group in the 1990s.

The final blow came this week, when Atlanta businesswoman Ginger White disclosed she had had a 13-year extramarital affair with Cain that ended only eight months ago.

Cain's support has been dropping as he has attempted to outlast the allegations and other campaign mishaps, but the bad news is taking its toll.

He said he would endorse another candidate at a later date but gave no hint of where he would direct his supporters to go.

Al Jazeera's Tom Ackerman, reporting from Washington, said: "It was clear Cain was falling in the polls when the allegations was made."

While Cain's ratings fell, support for former House of Representatives Speaker Newt Gingrich has surged.

Gingrich has now overtaken frontrunner Mitt Romney in some opinion polls on who should be the Republican candidate to challenge Barack Obama for the White House in November 2012.

"The bulk of Cain's supporters will go to Gingrich," rather than Romney, said our reporter.

Next month, voters in Iowa will begin the process of choosing a Republican presidential candidate for 2012.

No formal meaning

The phrase "suspending a campaign" has no formal meaning under US election law.

Candidates use it most frequently when they drop out of their primary race. The reason is that if a candidate "ended" the campaign instead of merely "suspending" it, then they might lose eligibility for federal matching funds that would help pay off their debts.

Used at least as far back as the 1970s, “suspending a campaign" remains the most popular way for candidates to end their bids without closing down their campaign panels.